This invent on relates to a non-aqueous liquid electrolyte secondary battery using a carbon material for its anode.
As technologies in electronics have been remarkably evolved, a variety of electronic equipment has become smaller and lighter. Accordingly, it has been required that batteries as portable power sources be increasingly smaller, lighter, and higher in energy density.
Conventionally, aqueous solution type secondary batteries, such as lead batteries and nickel-cadmium batteries, are primarily used as secondary batteries for general use. However, though these aqueous solution type secondary batteries exhibit excellent cyclic properties, they are not satisfactory in weight and energy density.
Meanwhile, recently, a non-aqueous liquid electrolyte secondary battery using lithium or a lithium alloy for its anode has been researched and developed prevalently. This non-aqueous liquid electrolyte a secondary battery has a high energy density, exhibits only a small amount of self-discharge, and is lightweight.
However, in this non-aqueous liquid electrolyte secondary battery, there is a high possibility that lithium is crystallized in dendritic form on the anode when the battery is charged in proceedings of charge/discharge cycles, and that the crystallized lithium finally reaches the cathode to generate an internal short circuit. Therefore, it is difficult to employ this non-aqueous liquid electrolyte secondary battery for practical use.
Thus, a non-aqueous liquid electrolyte secondary battery using a carbon material for its anode has been proposed. This non-aqueous liquid electrolyte secondary battery utilizes doping and releasing lithium into and from between carbon layers of the carbon material. By doing so, the battery does not exhibit such a phenomenon that dendritic lithium precipitates on the anode even though charge/discharge cycles proceed. The battery has a high energy density, and is lightweight as well as satisfactory in charge/discharge cyclic property.
Meanwhile, among various carbon materials usable for the anode of such an aqueous liquid electrolyte secondary battery, the first one that was practically used for the anode is cokes or a non-graphitizable carbon material, such as glass like carbon, that is, a carbon material of low crystalline property formed by heat-treating an organic material at a relatively low temperature. A non-aqueous liquid electrolyte secondary battery which has the anode formed of cokes or the non-graphitizable carbon material and a liquid electrolyte using propylene carbonate (PC) as the main solvent is already commercialized.
In addition, graphite of a high crystalline carbon material having a grown crystalline structure is recently used for the anode.
Graphite has a higher true density than the non-graphitizable carbon material of low crystalline property, and therefore exhibits a high electrode packing property when used for the anode. Thus, it is possible to design the battery of high energy density.
Graphite was considered to be unusable for the anode because it decomposes PC used as the main solvent of the conventional non-aqueous solvent. However, it has been found that the above inconvenience can be eliminated by using ethylene carbonate (EC) for the main solvent instead of using PC. Thus, a non-aqueous liquid electrolyte secondary battery which has the anode formed of graphite and a liquid electrolyte using EC as the main solvent has been proposed.
The non-aqueous liquid electrolyte secondary battery having the anode formed of graphite exhibits a high energy density and a flat discharge curve. Therefore, this battery is advantageous in that it generates no energy loss in voltage conversion by an electronic device.
However, the anode formed of graphite having a high true density causes lithium ions to diffuse slowly therein in the charge/discharge and is likely to cause polarization, while having high electrode packing property. For this reason if the battery is charged with relatively high drain, an overvoltage caused by polarization makes the anode potential baser than the lithium potential, causing lithium metal to precipitate on the anode surface. The precipitated lithium stays in a passive state, deteriorating the cyclic property.
If the non-aqueous liquid electrolyte secondary battery having the anode, formed of glass like carbon is charged at a constant voltage, which is the final voltage of 4.2 V of a practical battery the anode singly exhibits a potential of not higher than 50 mV vs. Li/Li.sup.+ at the end of the charge. On the other hand, if the non-aqueous liquid electrolyte secondary battery having the anode formed of graphite is charged under the same condition, the anode singly exhibits a potential reaching 100 to 150 mV at the end of the charge.
The non-aqueous liquid electrolyte secondary battery having the anode formed of graphite has a potential of the anode at the end of the charge 50 to 100 mV higher than the non-aqueous liquid electrolyte secondary battery having the anode formed of glass like carbon, though the two batteries are charged at the same final voltage. As the anode potential is high at the end of the cathode material, thus destabilizing the cathode. Consequently, charge, a large amount of lithium is extracted from the active the battery is not reliable in environment-resistant performance.